Polymers intrinsically show shape memory effects, e.g., on the basis of rubber elasticity, but with varied characteristics of strain recovery rate, work capability during recovery, and retracted state stability. Among the first shape memory polymers (SMP) reported as such was crosslinked polyethylene, which was discovered and patented in 1971 by Radiation Applications, Inc. and a methacrylic acid ester reported by the Vernon-Benshoff Co. and used as a denture material. However, the mechanism of strain recovery for such materials was immediately identified as far different from that of the shape memory alloys (SMAs), based largely on nickel-titanium alloys.
A shape memory polymer is actually a super-elastic rubber; when the polymer is heated to a rubbery state, it can be deformed under resistance of ˜1 MPa modulus, and when the temperature is decreased below either a crystallization temperature or a glass transition temperature, the deformed shape is fixed by the lower temperature rigidity while, at the same time, the mechanical energy expended on the material during deformation is stored. When the temperature is raised above the transition temperature (Tm or Tg), the polymer will recover to its original form as driven by the restoration of network chain conformational entropy. The advantages of the SMPs will be closely linked to their network architecture and to the sharpness of the transition separating the rigid and rubber states. Compared with SMAs, SMPs have an advantage of high strain (to several hundred percent) because of the large rubbery compliance while the maximum strain of a SMA is less than 8%. An additional benefit of the SMPs is that the transition temperature can be tailored according to the application requirement; e.g., tuning the transition temperature as thermal sensors and the triggered strain recovery above a predetermined temperature, e.g., 37° C. for biomedical applications.
Numerous polymers have been found to have particularly attractive shape memory effect, most notably the polyurethanes, the polynorbornene, styrene-butadiene copolymers, and crosslinked polyethylene.
Block copolymers of polystyrene (PS) and trans-polybutadiene (TPB) with a minor PS content offer an alternative approach to shape memory with a distinct mechanism of strain fixation and recovery triggering. While microphase-separated domains of the PS block are amorphous with Tg˜93° C., the continuous TPB phase is semicrystalline with Tg=−90° C. and Tm=68° C. Due to the immiscibility between PS and TPB blocks below 120° C., the copolymer forms a microdomain structure having elastic rheological characteristics above the TPB melting temperature, with the PS phase serving the role of physical crosslinking. Reversible deformations can therefore be fixed by crystallizing the TPB phase below about T=40° C. and recovered to the stress free state (shape memory) upon heating above 80° C. to melt the TPB phase and free the elastically deformed material to recover strain.
Another known semicrystalline shape memory polymer is trans-polyisoprene (TPI), having Tm=67° C., and degree of crystallinity near 40%, which readily undergoes crosslinking with peroxides. Below the Tm, the crosslinked TPI has a three dimensional network, which is connected by both chemical crosslinks and the crystalline regions. Above the Tm, the crystalline phase melts to become amorphous, with only the chemical crosslinks remaining to maintain the primary shape with a rubber-like modulus. This primary shape is the form of the material at the time of chemical crosslinking by peroxide cure, which normally occurs near T=145° C. for 30 minutes followed by cooling to room temperature, during which time crystallization occurs. As with the PS-TPB block copolymers, elastic deformation of crosslinked TPI can be carried out by heating the polymer above T=80° C. and this deformed secondary shape may be fixed by cooling-induced crystallization. The deformed shape returns to the primary form upon heating above 80° C.
In addition to the foregoing, copolymers of semicrystalline polycaprolactone (PCL) have been investigated as to their SMP characteristics. In particular, polycaprolactone diols have been difunctionalized with methacrylate end-groups and subsequently copolymerized with n-butyl acrylate. The polycaprolactone segments form a crystalline phase that can fix a secondary shape, while thermosetting leads to an elastic network that allows large reversible deformations above Tm. It was found that the PCL molecular weight controls the shape recovery temperature. It is believed that this is due to its influence on the melting transition, while n-butyl acrylate comonomer incorporation yields a softening effect due to the low glass transition temperature of poly (n-butyl acrylate) (Tg=−55° C.). It has been shown that the SMP based on polycaprolactone segments recovered their primary shape at 70° C. within 20 seconds, a relatively slow recovery.